A number of different methods for reducing the radiation dose to healthy tissue while maintaining the dose to the target during radiotherapy treatment exist. In the case of targets which undergo cyclical motion due to physiological movements of the patient, such as breathing, the sparing of healthy tissue is more complicated. There are various existing techniques to deal with moving targets. One approach is to gate the radiation, i.e. only allow the radiation beam to be active when the target is in an optimal position for treatment. In an alternative approach, the treatment beam may be widened to encompass the entire range of target positions, such that the target is continuously irradiated throughout its course of motion. In a yet further approach, the treatment beam may be continuously active, but targeted only at the average position of the tumour with the treatment beam(s) not fully encompassing the entire range of target positions.
Each of the previously mentioned techniques has its own drawbacks. Increasing the beam size to cover the entire range of motion obviously increases the amount of healthy tissue that will be exposed to radiation during the course of treatment. Although the target is continuously irradiated in this approach, the surrounding healthy tissue is also irradiated for a very large part of the movement cycle. Targeting the average tumour position reduces the dose applied to healthy tissue relative to the previous approach, but still more healthy tissue is exposed than is ideal.
Gating further reduces the amount of healthy tissue irradiated, and results in the least amount of healthy tissue irradiated of the three approaches described above. However, this requires that the beam is turned off for large periods of time during the treatment, leading to increased treatment times and lower patient throughput.
What is needed is a technique that combines the advantages of these techniques, with reduced disadvantages.